The present invention relates to lift trucks in general and, more particularly, to a braking system for a lift truck that permits simplified braking structure, elimination of replacement of brake wear components, improved operator comfort and improved lift truck stability during braking. While the present invention may be utilized on a variety of materials handling vehicles, it will be described herein with reference to a counterbalanced lift truck for which it is particularly applicable and will initially be used.
Oftentimes, lift trucks, particularly standup lift trucks, utilize a “black and white” or on/off spring applied/electrically released brake arrangement to provide both service brakes (brakes for slowing or decelerating a truck while moving) when an operator makes a pedal brake request, and also for parking brakes (brakes applied to immobilize a stopped truck). In these lift truck braking systems, limited modulation may be available by gradual application of the brake pedal and/or by pulsing the brake pedal to turn the brake on and off to thereby control the stopping distance. Further, the braking force applied by these braking systems typically is the same regardless of fork height and/or load carried by the forks. Furthermore, in these braking systems, during a “fault” condition within the truck control system, even the ability to modulate the brakes by pulsing the brake pedal is not available since full braking torque is applied regardless of whether the brake pedal is operated or not. Unfortunately, when a truck is operating with its forks elevated and particularly when the elevated forks support a load, full braking torque can result in an unstable condition.
Accordingly, there is a need for an improved braking system that addresses the noted properties in existing lift truck braking systems. Preferably, the improved braking system would provide substantially all service braking using regenerative or regen braking by appropriate control of motors used to drive the truck so that during braking, power is generated in the motors and conducted to the power system of the lift truck. In this way, operating times for the truck could be extended, mechanical braking systems used for parking and backup braking could be simplified and these braking systems normally would not require replacement or adjustment of wear components during the entire useful life of the lift truck. In addition, the mechanical brakes provided for parking and backup braking would be controlled so that an operator of the truck could modulate these brakes to improve stability of the truck during electrical system problems.